This invention relates to a vehicle transmission, particularly but not exclusively for material handling vehicles.
It is known to provide vehicle transmissions, particularly automatic transmissions, and otherwise mechanical transmissions for vehicles where engine load is likely to be highly variable such as material handling vehicles, with a torque converter. The torque converter provides a fluid coupling between the engine and the mechanical transmission which accommodates a variable load and also provides a degree of torque multiplication when the engine speed is higher than the transmission speed for example, when the vehicle is starting to move or is otherwise under load.
The presence of a torque converter can however be disadvantageous in some circumstances. At speed, where the engine speed generally matches the transmission speed, the torque converter only operates at 90 to 95% efficiency, at the maximum, the remainder of the power being converted to heat within the torque converter. When a vehicle is driven through a torque converter encounters, for example, a hill, the efficiency of the torque converter will fall as the difference between the engine speed and the vehicle speed increases, resulting in the vehicle slowing down in an undesirable manner.
To overcome these problems it is known to provide some means of locking the torque converter, such as an internal clutch as shown in our earlier application number GB 2389155. To solve the problem of the vehicle travelling along a road at a relatively high speed, it has also been proposed to bypass the torque converter to provide an additional top gear available when the highest gear transmission has already been selected, as shown in our earlier application number GB 2304834.
The known solutions are however not completely satisfactory. The use of an internal lock up clutch is constrained by the available volume within the torque converter casing, imposes significant manufacturing costs, and in general has a limited capacity since only one clutch plate is provided. It is possible to provide more plates with still greater manufacturing costs and complexity. In addition, the internal clutch must be operated by high fluid flows within the torque converter.
An aim of the present invention is to reduce or overcome one or more of the advance problems.